It is not possible to address the COM ports with the captions.
However as a work-around for this problem, before your actual code on your devices, you could continuously publish a particular value.
Simulink can search for these values across all COM ports and assign the COM ports accordingly.
For example, if you have an Arduino Uno and an Arduino Mega, you could publish '1' continuously from the Uno and '2' continuously from the Mega.
Simulink can sniff through all COM ports and look for a '1' or a '2', and assign the COM port to the device